Marine platform foundation structure

ABSTRACT

The invention relates to a marine platform foundation structure adapted to support an operating deck at an offshore site. The structure extends upwardly from the floor of a body of water to the surface, having the lower end operably connected to anchor means disposed at the ocean floor. Said anchor comprises one or more upstanding posts upon which the foundation structure is slidably registered. The connection therebetween permits the foundation structure to oscillate through a limited degree about the anchor in response to displacing forces, while maintaining said lower end horizontally stationary.

United States Patent 119] Mott MARINE PLATFORM FOUNDATION STRUCTUREInventor:

US. Cl ..61/46.5 Int. Cl ..B63b 21/50, E02b 17/00 Field of Search..6l/46.5, 46.6; 114/.5 D; 9/8

References Cited UNITED STATES PATENTS 6/1961 Ludwig ..6l/46.5 5/1962Nesbitt 12/1967 Koonce et a]. 6/1970 Blenkarn 2/1971 Blenkarn et al. 2/1971 Ryan 51 Jan. 16, 1973 FOREIGN PATENTS OR APPLICATIONS 1,389,2161965 France ..61/46.5

Primary Examiner-Jacob Shapiro Attorney-Thomas l-l. Whaley and Carl G.Ries [5 7] ABSTRACT The invention relates to a marine platformfoundation structure adapted to support an operating deck at an offshoresite. The structure extends upwardly from the floor of a body of waterto the surface, having the lower end operably connected to anchor meansdisposed at the ocean floor. Said anchor comprises one or moreupstanding posts upon which the foundation structure is slidablyregistered. The connection therebetween permits the foundation structureto oscillate through a limited degree about the anchor in response todisplacing forces, while maintaining said lower end horizontallystationary.

3 Claims, 6 Drawing Figures Pmmwm 16 Ian 3.710.580

SHEET 1 UF 3 PATENTEDJAH 16 I975 SHEET 3 BF 3 MARINE PLATFORM FOUNDATIONSTRUCTURE BACKGROUND OF THE INVENTION In the exploration and drillingfor sources of crude oil at offshore locations, it has become necessaryand will continue to be necessary to search and operate further from theshore. As a consequence, the depth of the water in which future drillingand producing operations will take place promises to be greater. Whilemuch of the basic offshore technology heretofore known by the industrycan be applied to deep water operating sites as well as to shallow watersites, the former introduces many problems not previously dealt with.

There exist today basically two types of marine platforms in current usefor offshore drilling. One consists of a rigidly positioned platformwhich embodies legs extending to the ocean floor and which are piledinto the floor or bear against a foundation mat. The second form ofplatform embodies the use of either the floating ship, orsemi-submersible space frame type floating structure which is eitheranchored or self-propelled to maintain a position over a desired site.The second type embodies the advantage of mobility in that it can bereadily moved. However, there is considerable difficulty in maintainingsaid platform a constant predetermined desired position above a wellhead, particularly under adverse weather conditions. Another pertinentfactor for consideration is that the floating type structures are unableto support a drilled well which is completed above the waters surface.

A further factor regarding the floating type ship or structure is thatthe drill string, together with any guide means utilized such as cables,chains and the like, which extend through the water, are subjected tolateral displacing forces in the form of ocean tides and currents.

Of the above mentioned platform types, by far the one achieving widestuse consists of a working deck that is retained in place over a desiredsite by a rigid structure. It has been 'found however that in deepwaters an elongated, rigid although buoyant structure, can be soanchored as to remain pivotally movable about the structure lower endwhile supporting a platform at the water's surface. Such a buoyantstructure is thus fastened to an anchoring device at a single pointwhereby the surface located platform is laterally movanection weakens orfails.

It is therefore one of the objects of the invention to provide anoffshore marine platform embodying a buoyant although rigid supportstructure which is operably connected to an anchoring member. A furtherobject is to provide a device of the type described wherein theanchoring means includes facilities for permitting restrained verticalmovement of the support structure with respect to the floor supportedanchor member. Still another object is to provide a structure of thetype described embodying an internal shock absorbing mechanism.

DESCRIPTION OF THE DRAWINGS In the drawings,

FIG. I is a side elevation view of a marine platform herein describedsubmerged in a body of water.

FIG. 2 is a side elevation view illustrating the step in the method forassembling a platform.

FIG. 3 is an enlarged cross-sectional view taken through line 3-3 inFIG. 1.

FIG. 4 is an alternate embodiment of the view shown in FIG. 3.

FIG. 5 is an enlarged segmentary view in partial cross-sectionillustrating a single column of the platform shown in FIG. 1, and

FIG. 6 is a segmentary view similar to FIG. 5 illustrating the platformsupport column in an offset position.

In overcoming the foregoing mentioned problems, and toward achieving thesuggested objectives, there is provided in brief a novel supportfoundation structure for an offshore platform adapted particularly todeep water operations. The unit includes an elongated superstructure orfoundation member 10 that is buoyantly stabilized in the body of waterto support a working deck 11 carrying equipment and materials necessaryto a drilling, producing or storage operation.

Anchoring means 12, resting at or partially imbedded in the ocean floor,includes means for operably engaging the lower end of the supportstructure at a slidable joint 13. Said support structure is thereby freeto oscillate within a limited are about the slidable joint. Restrainingmeans extending from the anchor means 12 to working deck 11, defines thedegree of mobility within which the structure can move. Said restrainingmeans includes a cable, chain, or similar non-elastic member provided atone end with a dampening mechanism.

Anchoring means 12 includes in essence a foundation pad 14, and mayfurther include a ballastable barge 15 which supplements the downwardforce exerted by the anchor. The anchor is normally transported to anoffshore drill site or installation, and lowered from barge 15 or asimilar work vessel to the ocean floor.

FIG. 1 illustrates one embodiment of the apparatus presentlycontemplated which includes working deck 11 disposed at, and preferablyabove the water's surface. Deck 11 carries such equipment and materialsas one or more derricks 9 together with draw-works, rotary table, andother features essential to the particular drilling operation.

Deck 11 is supported a desired distance above the water's surface by thebuoyant foundation structure 10 which includes one or more, andpreferably three relatively elongated column I6, 17 and 18 that columnsa generally vertical disposition in the body of water. The lower end ofeach of the respective columns is slidably received at a connector postsuch as 19, 20 and 21 protruding from the upper face of the anchoringmeans 12. The respective columns l6, l7 and 18 provide lateral supportfor one or more casing guides 22, shown in FIGS. 3 and 4, whichsimilarly extend upwardly through the water and enclose drill guideconduits extending longitudinally therethrough.

It is appreciated that the disclosed offshore platform together with itsfoundation structure, can be used readily in water of any depth andfunction in the desired manner. However, the device finds specificapplication in the instance of relatively deep water since the latterintroduces the problems which preclude the practical use of rigid, oceanfloor supported units Referring to FIGS. 1 and 2, anchoring means 12used for positioning the buoyant support structure 10, includesfoundation pad 14. The latter is equipped with one or more upright posts19, 20 and 21 usually formed of heavy walled steel or a similar highstrength material which can be prefabricated into pad 14 at a shorelocation and transported to a drilling site. Foundation pad 14 includesany number of such post members appropriately disposed to restrictmovement of superstructure l and to accommodate the respective columnsthereof to prevent the latter from rotating when submerged in the water.A preferable disposition of posts 19, 20 and 21 includes an arrangementthereof in recognition of the prevailing direction of the water currentsand storm winds at a particular site. Thus said posts are disposed insubstantially horizontal alignment.

FIGS. 5 and 6 illustrate anchoring posts 19 and 20 depending upwardlyfrom the surface of foundation pad 14. Each post such as 19, includes aheavy walled steel casing 24, having an open end 26 at one extremity,and a rounded closed end at the other. The casing open end is at leastpartially imbedded in concrete foundation pad 14 to form a firmconnection with the latter.

Anchoring posts 19 and 20 can be provided internally with a rigidmaterial such as a concrete filler 27, which may be further reinforcedas needed. Under operating conditions, anchoring posts 19 and 20 aresubjected to considerable bending stress while performing their functionof anchoring the elongated support column. The filler material affordsthe respective posts with additional bending resistance as well aslateral support for the casing wall.

The respective posts 19, 20 and 21 are interconnected by bracingelements extending therebetween such as heavy gauge 1 beams 29 or thelike similarly imbedded into the concrete base and so positioned toprovide the respective posts with a desired lateral spacing arrangement.Said members are thus rigidly positioned whereby to facilitate beingsupported by barge and subsequently lowered to a drilling site at theocean Support structure 10 includes elongated columns l6, l7 and 18which, in their normal upright position, extend from the anchoring means12 to and above the surface of the water. Each column, 16 for example,includes a central portion having internal tanks or other storage meansadapted to hold water for buoyancy purposes and oil for storagepurposes. While not presently shown, such buoyancy and storage tanks arewell known in the art and do not constitute a specific feature of theinvention.

The tanks at the upper end of each column 16, 17 and 18 are providedwith a controllable buoyancy system so disposed to afford a desireduplift to the latter depending on the weight of the platform deck 11.The column lower ends are provided with shaped means forming anelongated cavity 32 adapted to receive the upper end of an anchoringpost 19, 20 and 21 whereby to define a sliding yet operable engagementbetween the anchoring posts and the respective support column.

Said column lower end is formed with shell 33 having an annular bearingring 34 defining the inlet to cavity 32. Ring 34 is welded or otherwiseretained in place at the shell inner wall. The center opening of ring 34is of a sufficient diameter to slidably register about the outer wall ofthe anchoring posts without binding as the column assumes a canteddisposition. The connection thereby formed is both nonrigid, yetflexiblel Cavity 32 inlet defined by the rubbing ring 34 center opening,gradually widens along divergent walls of the shell 33, to a maximumdiameter at the shell upper end. Thereafter the diameter of internalcavity 32 is reduced along a frustro-conical section 36 whichterminates, and opens into the support column lower end.

Functionally, column 16 lower end is so arranged as to define a maximumangle of about 5 intermediate anchor posts 19, 20 and 21, and thedivergent wall cavity, whereby the entire column is permitted tooscillate about the lower guiding point in response to forces exertedagainst the column. Further, such movement is permitted regardless ofthe longitudinal relationship of the column with respect to the anchorpost. As presently shown, support structure 10 includes the threeadjacently positioned columns 16, 17 and 18 of the type described.However, it is appreciated that the number of such columns utilizeddepends on the size and weight of the platform deck whereby to properlysupport the latter at a desired height during a drilling operation.

Also included in foundation structure 10 are one or more elongatedtubular conduits extending substantially the length of structure 10.Said casings 23 and 24 are laterally supported by a plurality of easingguides 22. The latter comprise shaped cylindrical guide tubes ofsufficient diameter to accommodate a drilling conductor casing wherebyto guide the latter downwardly into the ocean floor. Cross members 38and 39 extending between the respective columns and guides 22,- serve torigidly position the columns with respect to each other.

As noted, the herein described connection 13 between the respectivecolumns and anchor posts, permits relative vertical movement betweensaid members as well as oscillatory motion of the column and deck aboutanchor 12. The column assembly however is restrained to movement withina limited degree of arcuate motion to preclude its becoming disconnectedfrom the anchor means.

Such restraining means includes an elongated cable 41 or similarrelatively non-elastic member, having the lower end firmly held in aretainer 42 disposed at the upper, inner wall of the respective anchorposts 19, 20 and 21. Cable 41 extends through an opening 43 in thecasing upper end and is carried to the top of column 16 and thence overpulley 56 where the end is retained in a dampening mechanism 46.

Exemplary of the latter, and as shown in FIG. 5, a reservoir is providedholding a quantity of transfer fluid within a compression cylinder 51.The latter guidably encloses a piston 52. Cable 41 is connected to thepiston end 53 whereby to exert tension on the latter at such time asconnection 13 at the column lower end tends to separate. Thus whenfoundation structure is subject to wind or water forces, the structurewill tend to displace laterally as well as vertically. Cable 41 howeverwill adjust whereby to pay out or increase tensional pull in response toadjustment of dampening mechanism 46.

Referring to FIG. 2, in a practical application of the disclosedapparatus and its method of use, anchor means 12 is initiallytransported to an offshore drill site. This is achieved most readily bycarrying anchor 14 on a barge or, if overly large, supported in asubmerged condition thus lessening the installation problem.

Anchor 14 is then controllably lowered to its desired ocean floorlocation by winches or other means such as a derrick barge 25, duringwhich time the ends of guide cables 41 connected to anchor posts 19, and21 are maintained at the waters surface by buoys 31 or th like.

To facilitate movement of foundation structure 10 to the site, the unitis made buoyant to the extent that it lies substantially horizontalat'the waters surface. In such condition it is readily towed or bargedas the circumstances require. Prior to the submerging of said foundationstructure at the drill site, the ends of guide cables 41 are threadedthrough ring 34, the length of the respective columns 16, 17 and 18, tothe upper end thereof. The buoyancy and attitude of the structure arethen adjusted such that the unit will assume a generally uprightdisposition in the water with the upper end protruding above thesurface. With the foundation structure 10 buoyantly positioned aboveanchor 14, buoyancy tanks in the respective columns are partially filledsuch that the unit will slowly and controllably descend through thewater guided by cables 41. The descent is further regulated throughupward pull of a crane barge or similar mechanism at the waters surface.

As the lower end of the structure reaches the anchor means 12, therespective column ends will register with the upstanding anchor posts19, 20 and 2l'through guide rings 34 to complete the nonrigidconnection. Thereafter, the upper ends of the guide cables 41 aresecured to dampening mechanism 46. To properly adjust the length ofguide cables 41, consideration must be given to the differential inwater depth at the drilling site, and also'the proposed variation inwater depth which will be reflected in the longitudinal travel of thering 34 along the anchor posts 19, 20 and 21.

Obviously, many modifications and variations of the invention, ashereinafter set forth, may be made without departing from the spirit andscope thereof, and therefore, only such limitations should be imposed asare indicated in the appended claims.

I claim:

1. A foundation structure for a marine platform having a work deck, andbeing disposed in an offshore body of water, said structure including;

a plurality of laterally joined elongated columns having opposed upperand lower ends, and a plurality of vertically spaced bracing membersinterconnecting said respective elongated columns, said respectivecolumns being floatably positioned in said body of water in a generallyupright disposition, the said column upper ends extending beyond thewater's surface and being adapted to supportably engage said work deck,

anchor means fixedly positioned at the floor of said body of water, andincluding a plurality of upstanding posts disposed in vertical alignmentwith said respective elongated columns, said posts having asubstantially uniform diameter along the outer surface thereof,

a shell forming a cavity at the column lower end thereof, said shellembodying an annular bearing ring having a center opening of asufficient diameter to slidably register about the said anchoring postand to maintain a sliding relationship therewith when said columnassumes a position canted from the vertical,

the lower of said column bracing members being disposed adjacent to saidshell forming said cavity,

whereby to permit vertical and pivotal movement of said respectivecolumns about their respective .anchor posts in response to lateraldisplacement of the column upper ends.

2. In a foundation structure as defined in claim 1 wherein saidupstanding anchor posts are formed with a cylindrical outer surface, andthe center opening in said bearing ring includes a circular openinghaving a diameter greater than that of said cylindrical outer surface.

3. In a foundation structure as defined in claim 1, including afrustro-conical section carried on said shell to define the upper endthereof, said conical section including a constricted portion connectedto and depending from said column lower end.

1. A foundation structure for a marine platform having a work deck, andbeing disposed in an offshore body of water, said structure including; aplurality of laterally joined elongated columns having opposed upper andlower ends, and a plurality of vertically spaced bracing membersinterconnecting said respective elongated columns, said respectivecolumns being floatably positioned in said body of water in a generallyupright disposition, the said column upper ends extending beyond thewater''s surface and being adapted to supportably engage said work deck,anchor means fixedly positioned at the floor of said body of water, andincluding a plurality of upstanding posts disposed in vertical alignmentwith said respective elongated columns, said posts having asubstantially uniform diameter along the outer surface thereof, a shellforming a cavity at the column lower end thereof, said shell embodyingan annular bearing ring having a center opening of a sufficient diameterto slidably register about the said anchoring post and to maintain asliding relationship therewith when said column assumes a positioncanted from the vertical, the lower of said column bracing members beingdisposed adjacent to said shell forming said cavity, whereby to permitvertical and pivotal movement of said respective columns about theirrespective anchor posts in response to lateral displacement of thecolumn upper ends.
 2. In a foundation structure as defined in claim 1wherein said upstanding anchor posts are formed with a cylindrical outersurface, and the center opening in said bearing ring includes a circularopening having a diameter greater than that of said cylindrical outersurface.
 3. In a foundation structure as defined in claim 1, including afrustro-conical section carried on said shell to define the upper endthereof, said conical section including a constricted portion connectedto and depending from said column lower end.